changing permission of folder in usb

I am trying to install owncloud on USB
I moved the installation on /mnt/usb/PirateBox/ and tried to install it,
but it gives error as

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Data directory (/mnt/usb/PirateBox/owncloud/data) is readable for other users
Please change the permissions to 0770 so that the directory cannot be listed by other users.

it require changing permission to 770 but its not working in openwrt,

there is no permission to change the directory permission, I searched for the issue and ca,e to know that external drive is needed to be mounted with option

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-o permission

but there is no option in mount command like permission,
how to do that ?